It is known to improve the quality of reproduction of audio frequency performances by increasing the signal-noise spacing by a compression-expansion system (so-called compander system). The useful signals are compressed in amplitude before the disturbed transmission path e.g. line or sound tape, and are expanded after transmission. In the case of companders for professional use an endeavour is made to obtain a fully constant slope of the compander and expander characteristics over as wide a range of regulation as possible. The advantage of such characteristics is that the input level of the expander may deviate from the output level of the compressor without the dynamic variation of the reproduced signal differing from the dynamic variation of the original signal.
This wide range of regulation desired for professional use cannot however be combined, with the compatibility required for consumer use, with transmission devices of constant amplification. The reproduction of such a compressed signal without expander has a poor sound since e.g. interference in pauses of the input signal is greatly increased. Likewise unsatisfactory is the audio impression with an uncompressed signal which is reproduced via an expander having an above-mentioned regulating characteristic. In this case a very much exaggerated dynamic of the reproduction occurs.